This work is concerned with the comparative, developmental, evolutionary and formal genetics of the species complex known as Tetrahymena pyriformis. Among the major projects in progress are; 1) the genetic organization of the macronucleus in T. thermophila, explored by means of assortment analyses of marconuclei constructed from primordia with various chromosomal compositions: hypodiploids, triploids, trisomics, etc., this project employs immunological markers because these are the only ones available in multiple allelic series; 2) the comparative genetics of T. pigmentosa, employing crosses of two subspecies with very great molecular diversity. The genetic markers used here are isozyme mobilities, immobilization antigens and mating types; 3) the nuclear determination associated with mating type fixation in T. thermophila, especially the timing of temperature influences on the frequencies of particular mating types; 4) the estimation of genetic and phenetic distances among the 19 species of the complex, as a means of reconstructing their evolutionary relations. In this analysis we have completed a survey of scalar and cytogeometric variation, and have begun a new exploration of enzyme mobilities.